Cardiac pacing electrically stimulates the heart when the heart's natural pacemaker and/or conduction system fails to provide synchronized atrial and ventricular contractions at appropriate rates and intervals for a patient's needs. Such bradycardia pacing provides relief from symptoms and even life support for hundreds of thousands of patients. Cardiac pacing may also give electrical overdrive stimulation intended to suppress or convert tachyarrhythmias, again supplying relief from symptoms and preventing or terminating arrhythmias that could lead to sudden cardiac death.
Cardiac pacing is usually performed by a pulse generator implanted subcutaneously or sub-muscularly in or near a patient's pectoral region. Implantable cardioverter-defibrillator (ICD) pulse generators usually include cardiac pacing functions, both for bradycardia support and for overdrive stimulation. The generator usually connects to the proximal end of one or more implanted leads, the distal end of which contains one or more electrodes for positioning adjacent to the inside or outside wall of a cardiac chamber. The leads have an insulated electrical conductor or conductors for connecting the pulse generator to electrodes in the heart. Such electrode leads typically have lengths of 50 to 70 centimeters.
A conventional pulse generator may be connected to more than one electrode-lead. For example, atrio-ventricular pacing, also commonly called dual-chamber pacing, involves a single pulse generator connected to one electrode-lead usually placed in the right atrium and a second electrode-lead usually placed in the right ventricle. Such a system can electrically sense heartbeat signals and deliver pacing pulses separately in each chamber. In typical use, the dual-chamber pacing system paces the atrium if no atrial heartbeat is sensed since a predetermined time, and then paces the ventricle if no ventricular heartbeat is sensed within a predetermined time after the natural or paced atrial beat. Such pulse generators can also alter the timing of atrial and ventricular pacing pulses when sensing a ventricular beat that is not preceded by an atrial beat within a predetermined time; that is, a ventricular ectopic beat or premature ventricular contraction. Consequently, dual-chamber pacing involves pacing and sensing in an atrium and a ventricle, and internal communication element so that an event in either chamber can affect timing of pacing pulses in the other chamber.
Recently, left-ventricular cardiac pacing has been practiced to ameliorate heart failure; a practice termed cardiac resynchronization therapy (CRT). CRT has been practiced with electrode-leads and a pulse generator, either an implantable cardioverter-defibrillator (CRT-D) or an otherwise conventional pacemaker (CRT-P). The left-ventricular pacing conventionally uses an electrode in contact with cardiac muscle in that chamber. The corresponding electrode-lead is usually placed endocardially in a transvenous manner through the coronary sinus vein, or epicardially. Left-ventricular pacing is usually practiced together with right-atrial and right-ventricular pacing with a single implanted pulse generator connected to three electrode-leads. CRT pulse generators can independently vary the time between an atrial event and right-ventricular pacing, and the time between an atrial event and left-ventricular pacing, so that the left ventricular pacing pulse can precede, follow, or occur at the same time as the right-ventricular pacing pulse. Similarly to dual-chamber pacing, systems with left-ventricular pacing also change atrial and ventricular pacing timing in response to premature ventricular contractions. Consequently, CRT-D involves pacing in an atrium and in two ventricles, sensing in the atrium and at least one ventricle, and an internal communication element so that an event in the atrium can affect timing of pacing pulses in each ventricle, and an internal communication element so that an event in at least one ventricle can affect timing of pacing pulses in the atrium and the other ventricle.
Pulse generator parameters are usually interrogated and modified by a programming device outside the body, via a loosely-coupled transformer with one inductance within the body and another outside, or via electromagnetic radiation with one antenna within the body and another outside.
Although more than one hundred thousand ICD and CRT-D systems are implanted annually, several problems are known.
A conventional pulse generator has an interface for connection to and disconnection from the electrode leads that carry signals to and from the heart. Usually at least one male connector molding has at least one additional terminal pin not required for defibrillation functions at the proximal end of the electrode lead. The at least one male connector mates with at least one corresponding female connector molding and terminal block within the connector molding at the pulse generator. Usually a setscrew is threaded in at least one terminal block per electrode lead to secure the connection electrically and mechanically. One or more O-rings usually are also supplied to help maintain electrical isolation between the connector moldings. A setscrew cap or slotted cover is typically included to provide electrical insulation of the setscrew. The complex connection between connectors and leads provides multiple opportunities for malfunction.
For example, failure to introduce the lead pin completely into the terminal block can prevent proper connection between the generator and electrode.
Failure to insert a screwdriver correctly through the setscrew slot, causing damage to the slot and subsequent insulation failure.
Failure to engage the screwdriver correctly in the setscrew can cause damage to the setscrew and preventing proper connection.
Failure to tighten the setscrew adequately also can prevent proper connection between the generator and electrode, however over-tightening of the setscrew can cause damage to the setscrew, terminal block, or lead pin, and prevent disconnection if necessary for maintenance.
Fluid leakage between the lead and generator connector moldings, or at the setscrew cover, can prevent proper electrical isolation.
Insulation or conductor breakage at a mechanical stress concentration point where the lead leaves the generator can also cause failure.
Inadvertent mechanical damage to the attachment of the connector molding to the generator can result in leakage or even detachment of the molding.
Inadvertent mechanical damage to the attachment of the connector molding to the lead body, or of the terminal pin to the lead conductor, can result in leakage, an open-circuit condition, or even detachment of the terminal pin and/or molding.
The lead body can be cut inadvertently during surgery by a tool, or cut after surgery by repeated stress on a ligature used to hold the lead body in position. Repeated movement for hundreds of millions of cardiac cycles can cause lead conductor breakage or insulation damage anywhere along the lead body.
Although leads are available commercially in various lengths, in some conditions excess lead length in a patient exists and is to be managed. Usually the excess lead is coiled near the pulse generator. Repeated abrasion between the lead body and the generator due to lead coiling can result in insulation damage to the lead.
Friction of the lead against the clavicle and the first rib, known as subclavian crush, can result in damage to the lead.
In dual-chamber pacing in an ICD, and in CRT-D, multiple leads are implanted in the same patient and sometimes in the same vessel. Abrasion between these leads for hundreds of millions of cardiac cycles can cause insulation breakdown or even conductor failure.
Subcutaneous ICDs that do not use endocardial, transvenous or epicardial lead wires, can deliver defibrillation using subcutaneous electrodes. However, pacing the heart from subcutaneous electrodes results in diaphragmatic stimulation which is uncomfortable to the patient if used in long-term therapy. Therefore pacing therapies such as bradycardia pacing therapy, anti-tachycardia therapy, atrial overdrive pacing for the prevention of arrhythmias, dual chamber pacing for atrio-ventricular synchronization and CRT therapies are inappropriate.